Shocking Spa..

[cubsno1]

I've been reading these forums for a while now and finally got my hot tub's water all balanced. Per the instruction on this forum and using measurements from Pool Calculator I started with Dichlor until my CYA was around 50ppm and have been using bleach ever since. With the understanding that I don't want to use a chlorine based shock I bought a non-chlorine shock / oxidizer (31% potassium peryxymonosulfate).

My question is this.. should "shocking" with this oxidizer raise my FC levels back up? Does it convert used chlorine back into free or does it just "evaporate" the used chlorine? If this is the case I must continue using bleach as the FC is being used? My 300g tub will sit covered and unused at 100 degrees for a few days and FC will go from 3-5ppm to 0.. how is the FC being used up or are my expectations incorrect and I need to be out there adding bleach every day (even when not being used)?

Thanks,
Tom

 

[harleysilo]

Hi Tom,

I don't know about any non-chlorine based shock. What I do is add a few oz. of bleach every day or every other day, depending on what my test and the pool calculator say i need. I've gotten it down to where I know i just add X number of oz of bleach and FC will be the right number. However my Hot tub is covered all the time.

I've found that when I use it i need to dose it right after as the FC gets used up with just and hour soak or so.

The constant addition of bleach is annoying, but my hot tub has NEVER looked better. Since it is recommended in the hot tub/spa section of pool school to change out the water every 3 months or so, and mine is only 250 gallons, i may just switch to using a stabilized chlorine in a floater since by the time CYA is interfering i can just dump/refill.

 

[chem geek]

You shouldn't need to shock the spa, just like you don't need to shock a pool, if you are properly using the right amount of chlorine. If you have no ozonator, then every person-hour of soaking requires around 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach or 7 teaspoons of non-chlorine shock (MPS), though after initial Dichlor, using bleach is the least expensive and most effective. If you use the right amount after every soak and add chlorine in between soaks to maintain a consistent level then you shouldn't ever need to shock. The goal is to add enough chlorine after your soak so that you have 1-2 ppm FC before your next soak, assuming it's the next day or two. That way you don't have a lot of noticeable chlorine during your soak (and I'm assuming a 30 ppm CYA level -- for 50 ppm CYA you might target 2-3 ppm FC minimum before the soak).

If you are only soaking once or twice a week, then you'll need to dose in between soaks. Dosing a little every day uses the least amount of chlorine, but for convenience if you wanted to dose larger amounts less frequently that's OK. In a spa without an ozonator, the usual daily FC loss in between soaks (i.e. after bather load is handled) is 25% per day. If you have an ozonator, then the amount of chlorine you need to add after a soak is cut down, usually in half, but the chlorine loss rate in between soaks is increased, usually doubled to 50%.

As for how often to replace the water, it doesn't have to be after 3 months. When using Dichlor-then-bleach, you can usually go twice as long vs. using Dichlor-only and even then the water is in better shape when you do end up changing it. With Dichlor-only, you usually have to change the water after cumulatively around 39 person-hours (in a 350 gallon spa -- 28 person-hours in a 250 gallon spa) while with Dichlor-then-bleach you can usually go to cumulatively 80 person-hours or longer. So if one is not using the spa every day, one can usually go longer than 3 months between water changes.

It is true that having to maintain the chlorine level is inconvenient, especially if one is not soaking every day or two. As for Trichlor pucks, they are not normally used in spas because they tend to dissolve too quickly in the hot water environment so you'd need to use a special feeder with exceptionally low water flow for this to work. Also, Trichlor is very acidic so you'll have to adjust pH though can target a higher TA level to help compensate with more carbon dioxide outgassing -- just like in a pool. At least with Trichlor, you'll build up the CYA level at 2/3rds the rate compared to Dichlor. If you go this route, let us know how it works out for you.

 

[cubsno1]

Can you explain why shocking a spa isn't necessary? How do you get rid of chloramines? It's my understanding that FC is "used" when it combines with sweat, body oils, etc. and this needs to be shocked and gassed off to avoid that chlorine odors. Is this accomplished through some other method? I don't have an issue with it now but am on a fairly fresh refill and have been using Dichlor and about to make the switch over to bleach. Just curious as it was my understanding that a spa needed to be superchlorinated or shocked (not sure these two terms are synonymous) to get rid of the used chlorine and/or convert it back to FC.

Thanks,
Tom

 

[chem geek]

You need to add a sufficient amount of chlorine right after your soak to handle the bather waste. If you want to call that shocking, that's fine, but here we usually refer to that term meaning raising the chlorine to a high level sustained for a period of time to handle some sort of problem such as killing algae that is already growing in a pool or clearing up water that has gone bad in a spa (usually because insufficient amounts of chlorine were used and it got too close to zero for too long).

If you properly maintain the spa and always have sufficient chlorine in it, then you don't need to shock. You will be adding required amounts of chlorine right after the soak such that you still have 1-2 ppm FC remaining at the start of your next soak (if you don't soak every day or two, then you need to add chlorine in between soaks to keep it higher so it still ends up at 1-2 at the start of your next soak). One person-hour will need roughly 7 ppm FC in 350 gallons to oxidize the bather waste, but I specified the amounts in quantities of chlorine in my previous post since it's independent of spa size.

The hot water in a spa has all chemical reactions go faster and in particular it seems to accelerate oxidation of urea which would otherwise be slow-to-oxidize. When you add chlorine after your soak, it immediately starts to oxidize the bather waste and you should keep your spa uncovered for an hour or so after your soak if you can. That lets the immediate by-products outgas. Chlorine will continue to oxidize bather waste (mostly urea) for a few hours but it should be complete long before your next soak even the next day. You should also uncover your spa for at least 10 minutes or so before you get into it to allow any accumulated gasses to dissipate as well.

You can, of course, measure the Combined Chlorine (CC) level just before your soak to see that in fact the bather waste seems to be getting handled. You can also monitor the chlorine demand to see that it isn't increasing over time which would indicate a buildup of unoxidized organics. This does happen slowly for the slowest-to-oxidize organics, especially if you are not clean when getting into the spa, so you may see the daily chlorine loss with no bather load go from 25% to 50% over many months until your next water change. These aren't organics that would oxidize quickly even if you shocked so there's not much point in doing that. It's just easier to change the water. At least with Dichlor-then-bleach you can go about twice as long between water changes and the water quality is still better when you change it. This is because there is no buildup of CYA which reduces the active chlorine level.

Note that in addition to initially using Dichlor for about a week to build up the CYA level to around 30 ppm or so you should also use Dichlor about one day per month to add some CYA that slowly gets oxidized during the month, usually at a rate of around 5 ppm per month.

 

[cubsno1]

That makes a lot more sense.. thank you for clarifying! It really helps.